It is known that triphenylphosphine, which is a product of the method of the present invention, is very useful in the synthetic organic chemical industry. For example, triphenylphosphine is used for producing olefines. Also, triphenylphosphine is useful for the synthesis of various useful organic compounds, for example, vitamin A and D. Furthermore, triphenylphosphine is useful for preparing heterocyclic nitrogen compounds by the deoxygenation of nitro and nitroso compounds.
When triphenylphosphine is used for the production of the above-mentioned compounds, the triphenylphosphine per se is converted to triphenylphosphine oxide, which is useless in the chemical industry. No economical method for regenerating triphenylphosphine from triphenylphosphine oxide has been known. However, it is known that triphenylphosphine oxide can be easily converted to a triphenylphosphine dihalide by treating it with a halogenating agent, for example, phosgene, oxalyl halides, thionyl halides and phosphorus pentahalides. Also, triphenylphosphine oxide can be converted to triphenylphosphine dichloride by treating it with chlorine and carbon monoxide in a solvent. This method is disclosed in Japanese Patent Application Laying-open (Kokai) No. 53-142999/1978.
As stated above, the use of triphenylphosphine for various organic chemical processes, results in the production of triphenylphosphine oxide as a by-product, and the triphenylphosphine oxide can be easily converted to a triphenylphosphine dihalide. Therefore, if an economical method for converting the triphenylphosphine dihalide to triphenylphosphine, were provided, the economical method would be remarkably valuable for developing the use of triphenylphosphine.
Also, it is knwon that triphenylphosphine is produced by reacting triphenylphosphine oxide with a silicon tetrahalide and pressurized hydrogen in a solvent, in the presence of sulphur or selenium, under an elevated pressure. This method is disclosed in British Pat. No. 1450830. Further, it is known from Japanese Patent Application Laying-open (Kokai) No. 53-34725/1978 that triphenylphosphine can be produced by reacting triphenylphosphine dihalide with pressurized hydrogen in an aromatic hydrocarbon solvent, for instance, toluene.
However, the above-mentioned former method for producing triphenylphosphine directly from triphenylphosphine oxide, is disadvantageous not only in that it is necessary to use silicon tetrahalide, sulphur and selenium, but also, in that a large amount of a by-product is produced from silicon tetrahalide, and a long time is necessary to complete the conversion. Accordingly, it is difficult to industrially utilize the above-mentioned method. Also, the above-mentioned latter method for producing triphenylphosphine from triphenylphosphine dihalide is disadvantageous in that the stoichiometrical conversion of triphenylphosphine dihalide to triphenylphosphine with a high degree of yield thereof can be effected only under an extremely high pressure of about 100 kg/cm.sup.2. Also, this method causes corrosive hydrogen halide to be generated during the reaction and, therefore, it is necessary that the reaction be carried out in a glass-coated reactor. Accordingly, it is difficult to utilize the above-mentioned method on an industrial scale.
Under the above-mentioned circumstances, it is strongly desired in the chemical industry to provide an economical method for producing triphenylphosphine under a remarkably lower pressure than 100 kg/cm.sup.2 G.
On the other hand, it is known from, for example, A. Maercker, "Organic Reactions", Vol. 14, 388 (1967) that halogenated hydrocarbons react with triphenylphosphine to form a phosphonium salt thereof. Therefore, it has seemed to most persons skilled in the art that halogenated hydrocarbon, for example, monochlorobenzene and dichlorobenzens, cannot be used as a solvent for the production of triphenylphosphine.